1. Field of the Invention
The present invention relates to the papermaking arts. More specifically the present invention relates to a papermaker's fabric for use in transfer belts and press fabric applications.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of the cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Press fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to support and to carry the paper product being manufactured through the press nips.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fill this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Contemporary press fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needled a batt of fine, non-woven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back and forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing one base fabric within the endless loop formed by another, and by needling a staple fiber batt through both base fabrics to join them to one another. One or both woven base fabrics may be of the on-machine-seamable type.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, papermaker's fabric manufacturers are required to produce press fabrics, and other papermaker's fabric to the dimensions required to fit particular positions in the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each press fabric must typically be made to order.
In response to this need to produce press fabrics in a variety of lengths and widths more quickly and efficiently, press fabrics have been produced in recent years using a spiral technique disclosed in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al., the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,360,656 shows a press fabric comprising a base fabric having one or more layers of staple fiber material needled thereinto. The base fabric comprises at least one layer composed of a spirally wound strip of woven fabric having a width which is smaller than the width of the base fabric. The base fabric is endless in the longitudinal, or machine, direction. Lengthwise threads of the spirally wound strip make an angle with the longitudinal direction of the press fabric. The strip of woven fabric may be flat-woven on a loom which is narrower than those typically used in the production of paper machine clothing.
The base fabric comprises a plurality of spirally wound and joined turns of the relatively narrow woven fabric strip. The fabric strip is woven from lengthwise (warp) and crosswise (filling) yarns. Adjacent turns of the spirally wound fabric strip may be abutted against one another, and the helically continuous seam so produced may be closed by sewing, stitching, melting, welding (e.g. ultrasonic) or gluing. Alternatively, adjacent longitudinal edge portions of adjoining spiral turns may be arranged overlappingly, so long as the edges have a reduced thickness, so as not to give rise to an increased thickness in the area of the overlap. Further, the spacing between lengthwise yarns may be increased at the edges of the strip, so that, when adjoining spiral turns are arranged overlappingly, there may be an unchanged spacing between lengthwise threads in the area of the overlap.
In addition, transfer belts relate to the transfer of a paper sheet between sections, or between elements of a section, such as the individual presses in a press section, of a papermachine. Transfer belts may be designed both to carry a paper sheet through a portion of a papermachine, and to allow for paper dewatering.
As noted above, the primary function of all papermaker's fabric is removal of water from the paper sheet. Further, criteria such as smooth surfaces and uniformity are important factors to be considered for a papermaker's fabric. The surface topography of papermaker's fabrics contributes to the quality of the paper product. Efforts have been made to create a smoother contact surface with the paper sheet. However, surface smoothness or pressure uniformity of a papermaker's fabric is limited by the topography resulting from the weave pattern and the filament physical properties underneath the needled batt. In a woven fabric (or knitted fabric), smoothness is inherently limited by the knuckles formed at the cross-over points of intersecting yarns. Thus, there is a need for fabrics with superior smoothness characteristics and uniformity.
The prior art includes melting non-woven materials in a papermaker's fabric, such as fiber batt material or a layer of spunbound materials. The location and placement of fibers in a needled batt are non-uniform and cannot be predicted nor repeated from fabric to fabric. The same is true if a film of “meltable” material is used as the film will flow, usually in the direction of a heat source. However, the flow has tended to be non-uniform; plus these films or “sheaths” of the prior art do little to mask the non-uniformity of pressure distribution attributed to the base fabrics that make up the support structure of the press fabric.
For example, U.S. Pat. No. 4,565,735 is a papermaker's felt with a compressive batt layer needled to one or both sides of a woven base layer. The batt layer(s) are formed from a mixture of at least two types of fibers. The first type is present in only small quantities and has a melting point lower than the remainder of batt layer(s) and the base fabric. The felt is heated to a temperature above this lower melting point and the first fibers melt to bond the remainder together and to the base fabric. While some localized improvement in fiber to fiber bonding has occurred, little is done to improve either surface smoothness or masking of the base fabric.
U.S. Pat. No. 4,830,915 is a paper machine wet-press felt having multiple layers of non-woven batt fibers alternating with layers of polymeric mesh interposed between them. The mesh layers have a lower melting point than the batt layers. The layers may be fixed by needling, sewing, heating or some combination of these. Each of the mesh layers is preferably a non-woven netting. During formation the felt may be heated to a temperature above the softening temperature of the polymeric mesh. However, it does not appear that the felt is heated to the melting point temperature of the polymeric mesh. As a result, smoothness and/or pressure uniformity may not be provided.
DE 297 06 427 U1 relates to a flexible band for use in paper machines. The band has at least one side which has an impermeable layer, and which features resilient compressibility comprising a fiber structure characterized in that the impermeable layer is formed by melting the fibers of the fiber structure on one side of the band. The fibrous layer or non-woven mat construction contains a predetermined portion of thermoplastically deformable threads, or hot melt adhesive thread. As previously mentioned, the location and placement of fibers in a non-woven mat are non-uniform and cannot be predicted nor repeated from fabric to fabric. Also disclosed are woven layers formed from so-called bicomponent threads. These threads are placed in a sheath of meltable polymers. Under the influence of heat, the above-mentioned sheath melts, while the thread itself is unaffected. The molten material so created is responsible for the adhesive connection. The thermoplastically deformable threads do not provide superior smoothness characteristics because only the sheath is melted and the core remains after melting.
Additionally, U.S. Pat. No. 5,298,124 (which is assigned to Albany International Corporation) describes a transfer belt, the teachings of which are incorporated herein by reference. The transfer belt may have a surface topography characterized by a pressure-responsive, recoverable degree of roughness, so that, when under compression in a press nip, the degree of roughness will decrease, thereby permitting a thin, continuous water film to be formed between the transfer belt and a paper sheet to bond the paper sheet to the transfer belt upon exit from the press nip. When the original degree of roughness returns sometime after exit from the nip, the paper sheet may be removed from the transfer belt, perhaps with the assistance of a minimal amount of vacuum or suction, to a permeable fabric, such as a dryer fabric.
The sheet transfer belt disclosed in U.S. Pat. No. 5,298,124 may comprise a reinforcing base with a paper side and a back side, and may have a polymer coating, which includes a balanced distribution having segments of at least one polymer, on the paper side. The balanced distribution takes the form of a polymeric matrix which may include both hydrophobic and hydrophilic polymer segments. The polymer coating may also include a particulate filler. The reinforcing base is designed to inhibit longitudinal and transverse deformation of the transfer belt, and may be a woven fabric, and, in addition, may be endless or seamable for closing into endless form during installation on the paper machine. The reinforcing base may have one or more fiber batt layers attached by needling to its back side. The fiber batt layer or layers, which may also be referred to as a needled web, are attached to the back side of the reinforcing base to control the impregnation of the polymer coating into the reinforcing base from the paper side during the manufacturing process. During the life of the transfer belt on a paper machine, the needled web protects the load-bearing yarns of the reinforcing base from damage by abrasion.